docs: Bump `rust-version` down to 1.65.0 #124

Job	Run time
build (stable)	32s
build (stable, --no-default-features --features=syntect-fancy)	34s
build (stable, --all-features)	29s
build (beta)	26s
build (beta, --no-default-features --features=syntect-fancy)	28s
build (beta, --all-features)	30s
	2m 59s

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